Solution phase method for making phase change materials

ABSTRACT

A method to form a phase change material (PCM). The method includes preparing a polymer solution by mixing an amount of a polymer in a solvent and mixing the polymer solution with an UiO-66 metal-organic framework (MOF) to form a composite. The polymer is a polyethylene glycol (PEG). The method further includes subjecting the composite to ultrasonic agitation and evaporating the solvent from the composite to form the PCM. After the evaporation of the solvent, particles of the PCM exhibit rounded octahedral structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. application Ser. No.17/751,125, now allowed, having a filing date of May 23, 2022.

STATEMENT OF PRIOR DISCLOSURE BY THE INVENTOR

Aspects of the present disclosure are described in H. Zahir; “HybridpolyMOF materials prepared by combining an organic polymer with a MOFand their application for solar thermal energy storage”; May 25, 2021;American Chemical Society Energy and Fuels, incorporated herein byreference in its entirety.

BACKGROUND

TECHNICAL FIELD

The present disclosure generally relates to a method of forming a phasechange material (PCM) for energy storage, and particularly to a methodto form a PCM based on metal-organic frameworks (MOFs).

DESCRIPTION OF THE RELATED ART

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Metal-organic frameworks (MOFs), also known as porous coordinationpolymers (PCPs), are generally classified as microporous materials,though mesopores with different sizes may be observed. MOFs includecationic metal ion clusters, known as Secondary Building Units (SBUs),which are bridged by organic anionic linkers forming polyvalentcoordinative bonds. MOFs exhibit a highly symmetrical, crystallineconstruction of the lattice structures, and very large specific surfacearea. High porosity at molecular level makes them useful forapplications such as gas storage, catalysis, absorption heattransformation, etc. For energy storing applications, there are someinherent advantages offered by MOFs such as structural diversity,functionality, tailorability, and versatility. The structures andproperties of MOFs can be easily tailored by changing the species,geometry, size, and functionality of the modules via pre-design orpost-synthetic modifications. Immobilization of active functionalmaterials on MOFs and creation of highly controllable nanostructures hasgained momentum for energy applications. Despite this, pure MOFsgenerally have a low thermal conductivity, and low latent heat, thusmaking them less effective for use in energy storage applications.

Phase Change Materials (PCMs), also known as latent heat storagematerials, have gained popularity in thermal energy storage field. Phasechange materials (PCMs) are substances which release or absorb a largeamount of energy during a phase transition, usually from solid to liquidand vice-versa, although it is also possible in principle to usesolid-solid phase changes, for example, where the substance goes fromconforming to one crystalline structure to conforming to another. PCMsstore latent heat and utilize the heat of fusion during the phasetransition. During phase changes, the temperature of the PCM remainsnearly constant, despite addition of more heat. Thus, the heat flowingthrough the PCM gets “stored” within the PCM itself and is referred toas latent heat. Any appreciable temperature increase is only observedafter the phase change is complete. Water, paraffin waxes, bee wax,glycerin, fatty acids, etc., are commonly known PCMs.

Though PCMs are useful energy storage materials, they suffer from anumber of drawbacks, for example, the loss of PCM by leakage, lowthermal conductivity, etc. Leakage poses a serious issue of environmentcontamination and low thermal conductivity limits their effectivenessfor solar energy storage.

Leakage issues can be overcome by employing shape-stabilization of PCMs.Recently, many research groups have reported that shape-stabilized PCMs(ss-PCMs), typically fabricated using a polymer as the functional phaseencapsulated in an inorganic support. Support materials usable toprevent the flow of liquid PCMs are expanded graphite (EG), expandedperlite (EP), vermiculite, high density polyethylene, styrene, butadieneand so on. To enhance the thermal conductivity of PCM, additivematerials such as EG, montmorillonite, pentaerythritol, melaminepolyphosphate are used. Normally, the PCMs, supporting materials, andadditive materials are melted and mixed fully together at hightemperature, and the formed compound, ss-PCM, is then cooled down andshaped in mold until it becomes solid.

However, presently available ss-PCMs still suffer from problems like lowsurface area, low pore volume, and complex preparation methods. Thethermal, physical, chemical, and mechanical properties of ss-PCMs areheavily dependent on the raw materials and synthesis processes.Moreover, the encapsulation efficiency is unsatisfactory and theparticle sizes need to be reduced.

PolyMOFs, formed using organic polymers as the organic component of theMOF lattice, retain the beneficial characteristics MOFs, such asenhanced porosity and crystallinity. It has been reported that polyMOFscan be developed by mixing PBDC-8A (PBDC-AB2 triblock copolymer) andPEG-2000-2% or PEG-4000-1%. The synthesized polyMOFs exhibit betterviscoelasticity, enhanced conductivity, improved coordinative ability,etc. However, this combination produces an aggregated octahedralmorphology. Block copolymers with a large PEG quantity hamper octahedraformation. Further, an interlaced morphology is observed for polyUiO-66prepared from PBDC-8A-PEGMnOMe.

The use of a PEG-MOF based ss-PCM for thermal energy storage has beenreported. A latent heat value of 120.53 J/g was achieved using theSA@Cr-MIL-101-NH₂ stabilized PCM composites. But the performanceparameters, charging and discharging values, thermal conductivity,seepage test results, and compatibility of the fabricated ss-PCM was notreported.

Feng et al. in their publication titled “Phase change in modified metalorganic frameworks MIL- 101 (Cr): Mechanism on highly improved energystorage performance” reported the composite MOF material MIL-101(Cr)-NH₂loaded with stearic acid and the thermal properties associated with itsphase transition. However, both Feng et al. and Luan et al. reported avery low latent heat value and did not mention any other importantstorage parameters.

In light of aforementioned shortcomings and gaps in the research area,there exists a need to provide a superior PCM with high storagecapacity, high latent heat, higher thermal conductivity, and improvedleakage characteristics. In addition, there exists a need to provide amethod of preparing such PCM. There also exists a need to provide amethod of preparing a polyMOF based PCM composite formed by acombination of at least one polymer and at least one MOF.

SUMMARY

In an exemplary embodiment, a method to form a phase change material(PCM) is disclosed. The method comprises preparing a polymer solution bymixing an amount of a polymer in a solvent and mixing the polymersolution with an UiO-66 metal-organic framework (MOF) to form acomposite. The polymer is a polyethylene glycol (PEG). The methodfurther comprises subjecting the composite to ultrasonic agitation andevaporating the solvent from the composite to form the PCM. After theevaporation of the solvent, particles of the PCM exhibit roundedoctahedral structures.

In some embodiments, the solvent is ethanol and the PEG has an averagemolecular weight from 4000 to 10000.

In some embodiments, the PEG for preparing the polymer solution has anaverage molecular weight of 6000.

In some embodiments, the PCM has a thermal conductivity of from 0.8 to0.9 W/mK and the PCM has an energy storage efficiency from 92% to 97%.

In some embodiments, the particles of the rounded octahedral structurehave a size in a longest dimension that ranges from 200 nm to 500 nm.

In some embodiments, the PCM is a shape stabilized phase change material(ss-PCM) having a PEG:UiO-66 weight ratio of from 0.5:0.2 to 1.0:0.2.The ss-PCM has a latent heat value of from 125 J/g to 175 J/g at aPEG:UiO-66 weight ratio of 0.5. Further, the ss-PCM has a thermalconductivity from 0.4 W/mK to 0.6 W/mK at a PEG:UiO-66 weight ratio of0.5.

In some embodiments, the particles of the rounded octahedral structurehave a BET surface area of from 750 m²/g to 1250 m²/g.

In some embodiments, the mixing of the polymer solution furthercomprises mixing carbon nanotubes (CNTs) with at least one of thepolymer solution, the MOF, or a combination of both the polymer solutionand the MOF to form a PCM containing carbon nanotubes.

In some embodiments, the method forms a PCM comprising the CNTs in anamount from 1 wt. % to 4 wt. % of the PCM.

In some embodiments, the PCM has an impregnation ratio of at least 60%.

In some embodiments, the PCM has a freezing temperature of at least 35°C. and a melting temperature of at least 55° C.

In some embodiments, the PCM has an impregnation efficiency of at least55%.

In some embodiments, the PCM has a heat storage efficiency of at least99%.

In some embodiments, the PCM has an energy storage ability of at least70%.

In some embodiments, the particles of the rounded octahedral structurehave a micropore volume from 0.25 cm³/g to 2.5 cm³/g.

In some embodiments, the particles of the rounded octahedral structurehave a pore diameter from 5 Å to 50 Å.

In some embodiments, the PCM has a latent heat in a freezing process ofat least 100 J/g.

In some embodiments, the PCM has a latent heat in a melting process ofat least 110 J/g.

In some embodiments, the mixing of the polymer solution furthercomprises using an amine group-containing compound with at least one ofthe polymer solution, the MOF, or a combination of both the polymersolution and the MOF.

In some embodiments, a PEG-UiO-66 PCM prepared by the method in whichthe particles of the PCM exhibit rounded octahedral structures.

The foregoing general description of the illustrative embodiments andthe following detailed description thereof are merely exemplary aspectsof the teachings of this disclosure and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this disclosure and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 shows PXRD plot depicting the spectra of (a) UiO-66, (b)PEG-6000, (c) PU-0.5 PCM samples, (d) PU-0.7 PCM samples, and (e) PU-1.0PCM samples, according to certain embodiments of the present disclosure;

FIG. 2 shows PXRD plot depicting the spectra of (a) PU-1000-0.5 PCMsamples, (b) PU-4000-0.5 PCM samples, and (c) PU-10000-0.5 PCM samples,according to certain embodiments of the present disclosure;

FIG. 3 shows FTIR spectra of (a) UiO-66, (b) PEG-6000, (c) PU-0.5 PCMsamples, (d) PU-0.7 PCM samples, (e) PU-1.0 PCM samples, according tocertain embodiments of the present disclosure;

FIG. 4A shows FESEM image of as-synthesized UiO-66 sample, according tocertain embodiments of the present disclosure;

FIG. 4B shows FESEM image of as-synthesized PU-0.5 PCM samples,according to certain embodiments of the present disclosure;

FIG. 4C shows FESEM image of as-synthesized PU-0.7 PCM samples,according to certain embodiments of the present disclosure;

FIG. 4D shows FESEM image of as-synthesized PU-1.0 PCM samples,according to certain embodiments of the present disclosure;

FIG. 5A shows FESEM image of as-synthesized PU-1000-0.5 PCM samples,according to certain embodiments of the present disclosure;

FIG. 5B shows FESEM image of as-synthesized PU-4000-0.5 PCM samples,according to certain embodiments of the present disclosure;

FIG. 5C shows FESEM image of as-synthesized PU-6000-0.5 PCM samples,according to certain embodiments of the present disclosure;

FIG. 5D shows FESEM image of as-synthesized PU-10000-0.5 PCM samples,according to certain embodiments of the present disclosure;

FIG. 6A shows FESEM image of as-synthesized PU-1000-0.5 PCM samples at ahigher magnification, according to certain embodiments of the presentdisclosure;

FIG. 6B shows FESEM image of as-synthesized PU-4000-0.5 PCM samples at ahigher magnification, according to certain embodiments of the presentdisclosure;

FIG. 6C shows FESEM image of as-synthesized PU-6000-0.5 PCM sample at ahigher magnification, according to certain embodiments of the presentdisclosure;

FIG. 6D shows FESEM image of as-synthesized PU-10000-0.5 PCM samples ata higher magnification, according to certain embodiments of the presentdisclosure;

FIG. 7A shows TEM image of as-synthesized UiO-66, according to certainembodiments of the present disclosure;

FIG. 7B shows TEM image of as-synthesized PU-0.5 PCM samples, accordingto certain embodiments of the present disclosure;

FIG. 8 shows melting-freezing DSC curves of (a) PU-0.5 PCM samples, (b)PU-0.7 PCM samples, (c) PU-1.0 PCM samples, (d) PU-1000-0.5 PCM samples,(e) PU-4000-0.5 PCM samples, (f) PU-10000-0.5 PCM samples, (g)PU-0.5-CNT 5 wt % PCM samples, and (h) PU-NH₂-0.5 PCM samples, accordingto certain embodiments of the present disclosure;

FIG. 9A shows nitrogen adsorption-desorption isotherm of theas-synthesized UiO-66, according to certain embodiments of the presentdisclosure;

FIG. 9B shows pore size distribution of the as-synthesized UiO-66,according to certain embodiments of the present disclosure;

FIG. 9C shows nitrogen adsorption-desorption isotherm of theas-synthesized PU-0.5 PCM samples, according to certain embodiments ofthe present disclosure;

FIG. 10 shows TGA curves of the PU-0.5 PCM samples, UiO-66, andPEG-6000, according to certain embodiments of the present disclosure;and

FIG. 11 shows melting-freezing DSC cycling curves of PU-0.5 PCM samples,according to certain embodiments of the present disclosure.

DETAILED DESCRIPTION

The definitions of the terms as used herein are as follows. Unlessspecified otherwise, these terms are used alone or in combination withanother term in the meaning as defined.

Throughout the specification and the appended claims, a given chemicalformula or name shall encompass all isomers (stereo and optical isomersand racemates) thereof where such isomers exist. Unless otherwiseindicated, all chiral (enantiomeric and diastereomeric) and racemicforms are within the scope of the disclosure. Many geometric isomers ofC═C double bonds, C═N double bonds, ring systems, and the like can alsobe present in the compounds, and all such stable isomers arecontemplated in the present disclosure. Cis- and trans- (or E- and Z-)geometric isomers of the compounds of the present disclosure aredescribed and may be isolated as a mixture of isomers or as separatedisomeric forms.

All processes used to prepare compounds of the present disclosure andintermediates made therein are considered to be part of the presentdisclosure. Both the free form and salts of products are within thescope of the disclosure. If so desired, one form of a compound may beconverted into another form. Further, a given chemical formula or nameshall encompass all conformers, rotamers, or conformational isomersthereof where such isomers exist. Different conformations can havedifferent energies, can usually interconvert, and are very rarelyisolatable.

The term “latent heat” as used herein, refers to the energy absorbed orreleased by a substance during a change in its physical state, whereinsaid change occurs without a corresponding change in its temperature.One or more substances having high latent heat may be used for energystorage which include acetic acid, acetone, alcohol (ethyl or methyl),aluminum, ammonia, aniline, benzene, bismuth, brass, carbon dioxide,carbon tetrachloride, cast iron, chromium, cobalt, copper, decane,dodecane, ethyl ether, ethylene glycol, glycerin, heptane, hexane, iron,manganese, naphthalene, nickel, octane, paraffin, phenol, platinum,silver, water, and zinc.

The term “PCM” or “Phase Change Material” as used herein, refers tothose substances which can absorb or release a large amount of latentheat, as defined herein, when they go through a change in their physicalstate, i.e., from one physical state to other physical state andvice-versa. Non-limiting examples of PCMs include paraffin-basedmaterials, acids, sugars, sulfates, chlorides, aluminum, copper, gold,iron, lead, lithium, silver, titanium, zinc, nitrates, hydroxides, fattyacids, alcohols, and glycols.

The term “ss-PCM” or “Shape Stabilized PCM” as used herein, refers toPCMs, as defined herein, which use porous matrices as support.Non-limiting examples of porous support matrices include mesoporouscarbon, mesoporous silica, graphene, carbon nanotubes, metal foams, andmetal-organic frameworks.

As used herein “metal-organic frameworks” or MOFs are compounds having alattice structure made from (i) a cluster of metal ions as vertices(“cornerstones”)(“secondary building units” or SBUs) which aremetal-based inorganic groups, for example metal oxides and/orhydroxides, linked together by (ii) organic linkers. The linkers areusually at least bidentate ligands which coordinate to the metal-basedinorganic groups via functional groups such as carboxylates and/oramines. MOFs are considered coordination polymers made up of (i) themetal ion clusters and (ii) linker building blocks. Preferably a UiO-66MOF is deployed according to certain embodiments of the presentdisclosure. The UiO-66 MOF is made up of [Zr₆O₄(OH)₄] clusters with1,4-benzodicarboxylic acid struts. The terephthalic acid can besubstituted with one or more alkyl groups, one or more carboxylic acids,or one or more phosphate groups. Non-limiting examples of suitable MOFsinclude UiO66-NH2, UiO-67, UiO-68, HKUST-1, LIC-1, CPL-2, Cu-MOF,Cu-TDPAT, Ni-MOF, Zr-MOF, Zn-TSA Fe-P-MOF, TMU-16-NH2 F-MOF-1, MOP-1,MOF-74, MOF-101, MOF-177, MOF-235, MOF-253, MOF-5, IRMOF-16, MIL-53,MIL-53(Al)-NH₂, MIL-88A, MIL-88-Fe, MIL-88B-4CH3, MIL-100-Fe, MIL-101,PCN-333-Al, ZIF-8, ZIF-67, and ZIF-90.

The term “PEG” or “Polyethylene glycol” as used herein, refers to acondensation polymer of ethylene oxide and water, preferably synthesizedusing a ring-opening polymerization of ethylene oxide to produce a rangeof molecular weights and molecular weight distributions. PEGs can besynthesized in linear, branched, Y-shaped, star, comb, or multi-armgeometries. Non-limiting examples of PEG include PEG-100, PEG-200,PEG-400, PEG-600, PEG-800, PEG-1000, PEG-2000, PEG-4000, PEG-6000,PEG-8000, PEG-10000.

The term “PolyMOF” or “Polymer-MOF” as used herein, refers to MOFssynthesized using organic polymers as the organic component of the MOFlattice. One or more polymers may be used in the polymer solution suchas alkylene glycol, e.g., ethylene glycol and/or propylene glycol,polyvinylpyrrolidone, or poly (N-(2-hydroxypropyl) methacrylamide)(PHPMA). Non-limiting examples of organic polymers include epoxies,phenolics, polyurethanes, polyimides, amino resins, bismaleimides,glycidyls, aliphatic amines, cycloaliphatic amines, polyaminoamides,anhydrides, isocyanates, and polylactides.

The term “PEG-MOF” as used herein, refers to polyMOFs, as definedherein, in which the polymer used as the organic component of the MOFlattice is a PEG organic polymer.

In the drawings, like reference numerals designate identical orcorresponding parts throughout the several views. As used herein, thewords “a,” “an” and the like generally carry a meaning of “one or more,”unless stated otherwise. Further, the terms “approximately,”“approximate,” “about,” and similar terms generally refer to ranges thatinclude the identified value within a margin of 25%, 20%, 10%, or 5%,and any values therebetween. Furthermore, the terms “equal to,”“substantially equal to,” and similar terms generally refer to rangesthat include the identified value within a margin of 75%, 80%, 85%, 90%,95%, or 100%, and any values therebetween.

As used herein, the terms “optional” or “optionally” means that thesubsequently described event(s) can or cannot occur or the subsequentlydescribed component(s) may or may not be present (e.g., 0 wt. %).

Aspects of the present disclosure are directed to a method to form aphase change material (PCM) comprising a combination of at least onepolymer and at least one MOF. Aspects of the present disclosure are alsodirected to a method to form a phase change material (PCM) comprising acombination of at least one polymer, at least one MOF, and carbonnanotubes (CNTs). In some examples, the aspects of present disclosureare directed to preparation of composite PCMs comprising at least oneorganic or inorganic phase change material, at least one MOF, andoptionally CNTs. The organic phase change material that may be utilizedin various aspects is preferably an organic polymeric material. Theinorganic phase change material that may be utilized in various aspectsis preferably a hydrated salt or a metallic material. Further aspects ofthe present disclosure are related to a polymer-MOF based PCM preparedby the methods disclosed herein.

In an aspect of the present disclosure, the method of preparation of aphase change material (PCM) includes preparation of a polymer solutionby mixing an amount of a polymer in a solvent; mixing the polymersolution with a metal-organic framework (MOF) to form a composite;subjecting the composite to agitation; and removing the solvent from thecomposite to form the phase change material (PCM).

The polymer solution may be prepared by mixing a suitable polymer withany compatible solvent. The polymer may be a paraffin- or anon-paraffin-based material. In a non-limiting example, theparaffin-based material may be a paraffin n-carbons material, wherein nrepresents number of carbon atoms present in the paraffin material andmay be greater than or equal to 14 and less than or equal to 34,preferably greater than or equal to 19 and less than or equal to 29, orpreferably 24. In another non-limiting example, the non-paraffin-basedmaterial may be selected from a group consisting of fatty acids,alcohols, and glycols. In a non-limiting example, the non-paraffin-basedmaterial is a fatty acid. In some examples, the fatty acid is caprylicacid, capric acid, lauric acid, myristic acid, palmitic acid, stearicacid, or the like. In another non-limiting example, thenon-paraffin-based material is an alcohol. In some examples, the alcoholis capric alcohol, lauryl alcohol, myristyl alcohol, cetyl alcohol,stearyl alcohol, or the like. In another preferred embodiment thenon-paraffin-based material is a glycol and/or a polyglycol. In someexamples, the glycol is a polyethylene glycol (PEG). PEG may be selectedfrom a group of PEG homopolymers within a defined weight averagemolecular weight range such as for example PEG-200, PEG-400, PEG-600,PEG-800, PEG-1000, PEG-2000, PEG-4000, PEG-6000, PEG-8000, PEG-10000,and the like. In certain embodiments, the PEG may be combined withanother polymer such as polyvinyl alcohol, poly(lactic-co-glycolicacid), poly(lactic-co-glycol), polylactic acid, polycaprolactone,polyglycolic acid, polybutylene terephthalate, or polyethyleneterephthalate.

The solvent utilized for preparing the polymer solution may be selectedbased on the choice of the polymer. In a non-limiting example, thesolvent may be a polar protic, a polar aprotic, or a non-polar solvent.Exemplary polar protic solvents that may be used include water,methanol, ethanol, isopropyl alcohol, acetic acid, etc. Exemplary polaraprotic solvents that may be used include dichloromethane,tetrahydrofuran, ethyl acetate, acetonitrile, dimethyl formamide,dimethyl sulfoxide, acetone, hexamethylphosphoric triamide, etc.Exemplary non-polar solvents that may be used include pentane, hexane,benzene, carbon tetrachloride, diethyl ether, xylene, toluene, aceticacid, chloroform, and ethyl acetate.

The MOF used in the method of the present disclosure is desired toexhibit at least one of the following characteristics: (a) large surfacearea, (b) high pore volume, (c) high storage capacity, (d) minimum masstransfer limitation, and (e) high stability especially hydrothermalstability. In an aspect of the present disclosure, the MOF is desired toexhibit characteristic (e) and at least one characteristic selected from(a) to (d). In another aspect of the present disclosure, the MOFexhibits all the aforementioned characteristics. In a preferredembodiment, the particles of the rounded octahedral structure may have aBET surface area from 750 m²/g to 1250 m²/g, a micropore volume from0.25 cm³/g to 2.5 cm³/g, and an energy storage efficiency of at least99%.

Exemplary MOFs suitable for use with the methods of the presentdisclosure include Zr-based MOFs like UiO-66, UiO-67, MOF-808, NU-1000,Al-based MOFs like MIL-96, MIL-100, MIL-110, Cu-based MOFs like HKUST-1,Cr-based MOFs, Cd-based MOFs, etc. Further, the MOF employed may carrypost-synthetic modifications (PSMs), enhancing its desired properties orfunctionalities. In a non-limiting example, the MOF may have beenmodified by (a) metal or organic ligand substitution strategy to obtainan MOF with mixed metal/ligands; or (b) chemical function decoration toobtain an MOF decorated and/or substituted with a functional group, forexample, but not limited to, an alkyl substituted-, aryl substituted-,an amine decorated-, a carboxylate decorated-, a nitro decorated, or abifunctional modified-MOF.

In an aspect of the present disclosure, the mixing further comprisesmixing carbon nanotubes (CNTs) with at least one of the polymersolution, the MOF, or a combination of both the polymer solution and theMOF. In another aspect of the present disclosure, the mixing furthercomprises using an amine group-containing compound with at least one ofthe polymer solution, the MOF, or a combination of both the polymersolution and the MOF. In another aspect of the present disclosure, themixing further comprises using CNTs and an amine group-containingcompound with at least one of the polymer solution, the MOF, or acombination of both the polymer solution and the MOF.

The composite obtained from mixing an MOF and a polymer, according tovarious aspects of the present disclosure, is further subjected toagitation. The agitation may be performed by any suitable methods knownfor example, but not limited to, mechanical, magnetic, or ultrasonic.Ultrasonication may be of any suitable power ranging from 20 W to 100 W,preferably 30 W to 90 W, preferably 40 W to 80 W, preferably 50 W to 70W, or 60 W. Ultrasonication may also be of any suitable time rangingfrom 10 minutes to 1 hour, preferably 20 minutes to 40 minutes, or 30minutes.

Further, the composite obtained after the mixing or agitation, isprocessed to remove or separate the solvent to isolate the phase changematerial (PCM). In a non-limiting example, the solvent may be removed orseparated using decanting, filtration, evaporation, vacuum drying,vacuum evaporation, vacuum concentration, heating, or a combinationthereof.

Now turning to a particular aspect of the present disclosure, a methodto form a PEG-UiO-66 based PCM is disclosed. The method includespreparation of a PEG polymer solution by mixing an amount of PEG polymerin a solvent; mixing the polymer solution with UiO-66 MOF to form acomposite; subjecting the composite to ultrasonic agitation; andevaporating the solvent from the composite to form the PCM.

In another aspect of the present disclosure, a method to form aPEG-UiO-66-CNT based PCM is disclosed. The method includes preparationof a PEG polymer solution by mixing an amount of PEG polymer in asolvent; mixing the polymer solution with an UiO-66 metal-organicframework (MOF) to form a composite; subjecting the composite toultrasonic agitation; and evaporating the solvent from the composite toform the PCM. Carbon nanotubes (CNTs) are mixed with the solvent andpolymer when forming the PEG polymer solution. In other embodiments thecarbon nanotubes are mixed with at least one of the polymer solution,the MOF, or a combination of both the polymer solution and the MOF. Theformed PCM may comprise CNTs in an amount from 1 wt. % to 4 wt. % of thePCM, preferably at 1.5 wt. % to 3.5 wt. % of the PCM. Preferably 2 wt. %to 3 wt. % of the PCM, or 2.5 wt. % of the PCM.

The solvent employed for preparing the PEG polymer solution in any ofthe aforementioned aspects may be chosen from any of the above-mentionedexemplary solvent types. In some examples, the solvent is a polar proticsolvent. In some examples, the solvent used is ethanol.

Further, the PEG polymer used for preparing the polymer solution, asdescribed in earlier aspects of the present disclosure, may be of anysuitable average molecular weight ranging from as low as 200 to as highas 20,000. In some examples, the PEG has an average molecular weightranging from 2,000 to 10,000. In some examples, the PEG has an averagemolecular weight ranging from 4,000 to 8,000. In some examples, the PEGhas an average molecular weight of about 6,000.

The particles of the PCM obtained by the methods of the presentdisclosure exhibit rounded octahedral structures. As depicted in FIG.6C, the PCM may contain a first particle size distribution of octahedralPCM particles and a second particle size distribution of unreacted PEGsuch that an aggregated octahedral morphology is formed. The aggregatedoctahedral morphology is an intermediate structure of the unsynthesizedPCM. As depicted in FIG. 7A, the PCM is now fully synthesized and thePCM particles have an octahedral shape, displaying eight triangularfaces on the outside surface. The octahedral PCM particles are alsorounded, as depicted in FIG. 7A, where the top and bottom tips of theparticles are worn down to the point where the tips display a roundedcurvature at the tip instead of a sharp triangular pattern. Theroundedness can be attributed to having a circular or cylindricalcross-section at each tip of the particle, without the entire particletaking on a spherical shape due to its octahedral nature. Theroundedness can be further described by having a perfect octahedronwhere a face dimension is 1 cm, but an octahedron with a rounded edgehas a longest dimension that is 0.9 cm preferably 0.95 cm due to removalof the apex of the octahedron thereby shortening the face dimension andleaving a hemispherical shape at the location of the apex. The size ofthe octahedral structures increases with an addition of the PEG athigher molecular weights. In a non-limiting example, the particles ofthe rounded octahedral structure may have a size in a longest dimensionthat ranges from 200 nm to 500 nm, preferably 225 nm to 475 nm,preferably 250 nm to 450 nm, preferably 275 nm to 425 nm, preferably 300nm to 400 nm, preferably 325 nm to 375 nm, or 350 nm. In anothernon-limiting example, the particles of the rounded octahedral structuremay have a BET surface area from 750 m²/g to 1250 m²/g, preferably 800m²/g to 1200 m²/g, preferably 950 m²/g to 1150 m²/g, preferably 1000m²/g to 1100 m²/g, or 1050 m²/g. In another non-limiting example, theparticles of the rounded octahedral structure may have a microporevolume ranging from 0.25 cm³/g to 2.5 cm³/g, preferably 0.5 cm³/g to 2cm³/g, preferably 0.5 cm³/g to 1.5 cm³/g, 0.6 cm³/g to 0.8 cm³/g, or 0.7cm³/g. In yet another non-limiting example, the particles of the roundedoctahedral structure may have a pore diameter from 5 Å to 50 Å,preferably 10 Å to 45 Å, preferably 15 Å to 40 Å, preferably 20 Å0 to 35Å, preferably 25 Å to 30 Å, or 27.5 Å.

In some aspects, the PCM obtained by the methods of the presentdisclosure is a shape stabilized phase change material (ss-PCM). Thess-PCM may have a PEG to UiO-66 weight ratio ranging from 1:0.2 to0.2:1, preferably 0.8:0.4 to 0.4:0.8, or 1:1. In a non-limiting example,the PEG to UiO-66 weight ratio may be such that the PEG is at least 5times, at least 4.5 times, at least 4 times, at least 3.5 times, atleast 3 times, at least 2.5 times, at least 2 times, or at least 1.5times the UiO-66. In another non-limiting example, the PEG to UiO-66weight ratio may be such that the UiO-66 is at least 5 times, at least4.5 times, at least 4 times, at least 3.5 times, at least 3 times, atleast 2.5 times, at least 2 times, or at least 1.5 times the PEG.

In some examples, the ss-PCM may have a PEG to Ui0-66 weight ratioranging from 0.5:0.2 to 1:0.2, preferably 0.4:0.3 to 0.75:0.25, or 1:1.In some examples, the PEG to Ui0-66 weight ratio may be 0.5:0.2,preferably 0.45:0.25, preferably 0.4:0.3, or 1:1. In some examples, thePEG to Ui0-66 weight ratio may be 0.7:0.2, preferably 0.65:0.25,preferably 0.6:0.3, preferably 0.55:0.35, preferably 0.5:0.4, or 1:1. Insome examples, the PEG to UiO-66 weight ratio may be 1:0.2, preferably0.9:0.3, preferably 0.8:0.4, preferably 0.7:0.5, or 1:1. In someexamples, the PEG to Ui0-66 weight ratio may be 0.5:1, preferably0.6:0.9, preferably 0.7:0.8, or 1:1. In some examples, the PEG to UiO-66weight ratio may be 1:1.

The PCM may have a thermal conductivity ranging from 0.8 W/mK to 0.9W/mK, preferably 0.81 W/mK to 0.89 W/mK, preferably 0.82 W/mK to 0.88W/mK, preferably 0.83 W/mK to 0.87 W/mK, preferably 0.84 W/mK to 0.86W/mK, or 0.85 W/mK. In a non-limiting example, the PCM may have athermal conductivity ranging from 0.4 W/mK to 0.6 W/mK at a PEG toUiO-66 weight ratio of 0.5, preferably 0.42 W/mK to 0.58 W/mK,preferably 0.44 W/mK to 0.56 W/mK, preferably 0.46 W/mK to 0.54 W/mK,preferably 0.48 W/mK to 0.52 W/mK, or 0.5 W/mK.

Further, the PCM may have an energy storage efficiency of at least 99%,preferably at least 98%, 97%, 96%, 95%, 94%, 93%, 92%, 91%, or 90% andheat storage efficiency of at least 99%, preferably at least 98%, 97%,96%, 95%, 94%, 93%, 92%, 91%, or 90%. In a non-limiting example, the PCMmay have an energy storage efficiency from 92% to 99%, preferably 93% to98%, preferably 94% to 97%, preferably 95% to 96%, or 96.5%. In someexamples, the PCM may have an energy storage ability of at least 70%,preferably at least 68%, 66%, 64%, 62%, 60%, 58%, 56%, 54%, 52%, or 50%.

Furthermore, the PCM may have a latent heat in a freezing process of atleast 100 J/g, preferably at least 98 J/g, 96 J/g, 94 J/g, 92 J/g, 90J/g, 88 J/g, 86 J/g, 84 J/g, 82 J/g, or 80 J/g and a latent heat in amelting process of at least 110 J/g, preferably at least 108 J/g, 106J/g, 104 J/g, 102 J/g, 100 J/g, 98 J/g, 96 J/g, 94 J/g, 92 J/g, or 90J/g. In a non-limiting example, the ss-PCM may have a latent heat valuefrom 125 J/g to 175 J/g at a PEG to UiO-66 weight ratio of 0.5,preferably 130 J/g to 170 J/g, preferably 135 J/g to 165 J/g, preferably140 J/g to 160 J/g, preferably 145 J/g to 155 J/g, or 150 J/g.Additionally, the PCM may have a freezing temperature of at least 35°C., preferably at least 34° C., 33° C., 32° C., 31° C., 30° C., 29° C.,28° C., 27° C., 26° C., or 25° C. and a melting temperature of at least55° C., preferably at least 54° C., 53° C., 52° C., 51° C., 50° C., 49°C., 48° C., 47° C., 46° C., or 45° C.

In some examples, the PCM may have an impregnation ratio of at least60%, preferably at least 59%, 58%, 57%, 56%, 55%, 54%, 53%, 52%, 51% or50% and an impregnation efficiency of at least 55%, preferably at least54%, 53%, 52%, 51%, 50%, 49%, 48%, 47%, 46% or 45%. The impregnationratio can be defined as the amount of polymer that is absorbed into thess-PCM per unit volume of the ss-PCM. The impregnation efficiency isdefined as the amount of polymer absorbed into the ss-PCM per unitvolume of the ss-PCM per the amount of polymer used in total.

EXAMPLES

The following examples describe and demonstrate exemplary embodiments ofthe method to form the PCM described herein. The examples are providedsolely for the purpose of illustration and are not to be construed aslimitations of the present disclosure, as many variations thereof arepossible without departing from the spirit and scope of the presentdisclosure.

Materials

1,4-Benzenedicarboxylic acid, 2-amino-1,4-benzenedicarboxylic acid, DMF,and zirconium tetrachloride were used. Polyethylene glycol (MW 1000,4000, 6000, and 10000) used as the PCM material and solvent ethanol wereused as well. All chemicals were used without further purification.

EXAMPLE 1

Synthesis of UiO-66

UiO-66 was synthesized using a Hydrothermal Method.1,4-Benzenedicarboxylic acid (100 mg, 0.6 mmol), and ZrCl₄ (133.6 mg,0.6 mmol) were dissolved in 40 mL of DMF and heated at 393 K for 24 h toform the UiO-66 PCM. The resulting UiO-66 was washed three times withDMF (5-10 mL) using a centrifuge (10000 rpm for 30 min), and then wassequentially treated with methanol (5-10 mL three times per day) forthree 24 h periods. Finally, UiO-66 was activated by removing thesolvent under vacuum for 24 h at 100° C.

EXAMPLE 2

Synthesis of UiO-66-NH₂

UiO-66-NH₂ was synthesized by dissolving ZrCl₄ (125 mg, 0.54 mmol) and2-amino-1,4-benzenedicarboxylic acid (134 mg, 0.75 mmol) in DMF (20 mL)with ultrasonication for 30 min. The as-obtained mixture was transferredto a stainless-steel Teflon-lined autoclave of 50 mL capacity and heatedat 393 K for 24 h. Then the autoclave was cooled in air to roomtemperature. The resulting solid was filtered, repeatedly washed withCHCl₃, and dried at room temperature.

EXAMPLE 3

Preparation of composite PCM

PEG-MOF, specifically PEG-UiO-66 PCM composite, was synthesized bydissolving varying amounts of PEG-6000 (1.0 g, 0.7 g, and 0.5 g) and 0.2g of UiO-66 in 50 mL ethanol. PEG-6000 and UiO-66 were mixed well bystirring for 30 min. The mixture was then placed in an ultrasonicationmachine for another 30 min for further dissolution. The solution wasallowed to stand at 80° C. for 24 h with stirring to remove the ethanolby evaporation. Finally, PEG-6000-1.0 g/UiO-66-0.2 g (PU-1.0),PEG-6000-0.7 g/UiO-66-0.2 g (PU-0.7), and PEG-6000-0.5 g/UiO-66-0.2 g(PU-0.5) composite PCM samples were obtained.

In addition, composite PCM samples having PEG of different molecularweights, viz., PEG-1000-0.5/UiO66-0.2 g (PU-1000-0.5),PEG-4000-0.5/UiO66-0.2 g (PU-4000-0.5), and PEG-10000-0.5/UiO66-0.2 g(PU-10000-0.5) were also prepared.

Further, composite PCM samples having CNTs, PEG-0.5 g/UiO-66-0.2 g-CNT 5wt. % (PU-0.5-CNT 5 wt. %), and -NH₂ functional groups, PEG-0.5g/UiO-66-NH₂-0.2 g (PU-NH₂-0.5) were also prepared.

The PCM samples prepared using PEG-6000 (PU-1.0, PU-0.7, PU-0.5) werethen characterized, as described below. The properties of additionallyprepared PCM samples (PU-1000-0.5, PU-4000-0.5, PU-10000-0.5, PU-0.5-CNT5 wt. %, and PU-NH₂-0.5) were also evaluated for comparison purposes.

EXAMPLE 4

Powder X-Ray Diffraction (PXRD) pattern analysis

The PXRD patterns were obtained using a Bruker D8 advance diffractometersystem (Berlin, Germany). All data were collected at a scanning velocityof 3 min⁻¹ in the range of 2θ=10-70°.

Powder X-ray diffraction (PXRD) patterns are shown FIG. 1 and FIG. 2 .Referring to FIG. 1 , (a) shows the PXRD pattern of UiO-66 alone, (b)shows the PXRD pattern of PEG-6000 alone, (c) shows the PXRD pattern ofPU-0.5, (d) shows the PXRD pattern of PU-0.7, and (e) shows the PXRDpattern of PU-1.0. Referring to FIG. 2 , (a) shows the PXRD pattern ofPU-1000-0.5, (b) shows the PXRD pattern of PU-4000-0.5, and (c) showsthe PXRD pattern of PU-10000-0.5.

As seen in FIG. 1 (c-e) and FIG. 2 (a-c), all composites show similarPXRD patterns. The peaks appearing in the 2θ range of 15°-30° aresimilar to the diffraction peaks of crystalline PEG-6000 shown in FIG.1(b). Intense sharp peaks present at 2θ of 19.24° and 23.42° indicatethe presence of crystalline PEG-6000. The characteristic peaks at2θ=7.78° and 8.92° for UiO-66 with a high crystallinity was observed.

As seen in FIG. 1(b) and FIG. 1(c), the peaks of PU-0.5 are smaller thanthose of PEG-6000 alone, indicating that the pores of PU-0.5 areoccupied by the melted PEG-6000. The confinement of PEG-6000 melted inthe pores of the composites decreased the crystallite size of PEG-6000.

Further, as seen in FIG. 1(c), FIG. 1(d), and FIG. 1(e), PU-0.5 showsthe largest decrease in peak height compared to PU-0.7 and PU-1.0. Thisindicates that the liquid PEG-6000 retention in a PU-0.5 is best and alarger fraction of PEG-6000 is impregnated in the porous structure,implying that the PU-0.5 sample may provide excellent performance insolar energy storage. New peaks did not appear in the PXRD pattern shownin FIG. 1(c) when PEG-6000 is mixed with UiO-66, indicating that mixingis purely physical without affecting the crystallinity of PEG-6000 andUiO-66. Hence, UiO-66 MOF is a promising matrix to be used as a supportfor ss-PCM preparation.

Turning to FIG. 2 , the PXRD patterns of (a) PU-1000-0.5, (b)PU-4000-0.5, and (c) PU-10000-0.5, respectively, also show that the mainpeaks of PEG-6000 and UiO-66 MOF, as depicted in FIG. 1(a) and FIG.1(b), indicating that their mixing is purely physical.

EXAMPLE 5

Fourier-Transform Infrared (FTIR) spectroscopy

A Bruker FTIR spectrometer (Berlin, Germany) was used to record the FTIRspectra.

Referring to FIG. 3 , (a) shows the FTIR spectrum of UiO-66 alone, (b)shows the FTIR spectrum of PEG-6000 alone, (c) show the FTIR spectrum ofPU-0.5, (d) show the FTIR spectrum of PU-0.7, and (e) show the FTIRspectrum of PU-1.0.

In FIG. 3(a), FTIR spectrum of UiO-66 reveals bands in the range of1550-1630 cm⁻¹ and 1450-1580 cm⁻¹ which are due to the symmetric andasymmetric stretching associated with the carboxylate ligands. The broadpeak in the range of 3400-3450 cm⁻¹ is due to O—H stretching associatedwith the water molecules. The medium intensity peak at 745 cm⁻¹ is dueto Zr—O stretching. The peak at 728 cm⁻¹ is also due to M—O stretchingin UiO-66.

In FIG. 3(b), the peak at 1109 cm⁻¹ of PEG-6000 is attributed to thestretching vibration of C—O—C. The peak at 1095 cm⁻¹ is assigned toC—O—H, whereas the one at 1279 cm⁻¹ is attributed to OH. The two peaksat 1339 and 1464 cm⁻¹ are due to bending vibrations of C—H. Theabsorption bands at 2882 cm⁻¹ are due to the stretching vibrations ofC—H and OH, respectively.

The spectra of the composites shown in FIG. 3 (c-e) have peaks similarto those of PEG-6000, as shown in FIG. 3(b). The peak observed at awavenumber of 713.6 cm⁻¹ in FIG. 3(b) is also present in the spectra ofthe composites shown in FIG. 3 (c-e), indicating that the support matrixUiO-66 remains unchanged. Very intense peak at 882 cm⁻¹ is due to thestretching vibration associated with the functional group —CH2.

The FTIR spectra of PU-0.5, PU-0.7, and PU-1.0 show that peaks due toUiO-66 as well as PEG-6000 are present. No new peaks are present,indicating that only physical mixing is taking place in well-mixedUiO-66 and PEG-6000 composites. When seen in combination with PXRDpatterns, FTIR results indicate that PEG-6000 successfully penetratesthe porous structure of UiO-66, without disturbing the crystallinestructure of the MOF framework.

EXAMPLE 6

Morphology of the composite PCM samples was analyzed using FieldEmission Scanning Electron Microscope (FESEM) imaging and TransmissionElectron Microscope (TEM) imaging. FESEM (TESCAN LYRA3, Czech Republic)was used to determine the morphology and size of the particles. TEMimages were obtained using a JEOL Inc., JEM 2011, with a CCD cameraoperating at 200 kV.

FIG. 4A shows the FESEM image of UiO-66 alone. FIG. 4B, FIG. 4C, andFIG. 4D show the FESEM images of PU-0.5, PU-0.7, and PU-1.0,respectively.

As seen in FIG. 4B, FIG. 4C, and FIG. 4D, presence of two types ofparticles is revealed. Some particles have the morphology of originalMOF, i.e., UiO-66, while other particles are smaller but have a shapesimilar to that of UiO-66. These smaller particles are PEG-MOFs (PU-0.5,PU-0.7, PU-1.0) consisting of intergrown rounded octahedral structures.

As seen in FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D, the addition ofPEG-6000 to MOF yields small-sized PEG-MOFs with octahedral structuresand the amount or production yield of PEG-MOFs depends on the quantityof PEG-6000 added into the system. The formation of the smallest PEG-MOFparticles and the highest amount of PEG-MOF was observed in the case ofPU-0.5, as shown in FIG. 4B. The results indicate that PEG-6000penetrates the UiO-66 MOF and produces PEG-MOFs. Addition of a largerquantity of PEG-6000 into UiO-66 produces a lower quantity of PEG-MOFswith a thin-film type morphology, where the PEG-MOFs are attached toeach other very closely, as seen in FIG. 4C and FIG. 4D. Referring toFIGS. 4C and 4D, the penetration of PEG into the MOF can be seen, wherethe octahedral particles intertwine with the PEG during formation.

Referring to FIG. 1 , FIG. 2 and FIGS. 4A to 4D, the crystallinity ofthe PCM samples determined by PXRD can be explained using the morphologyof these samples. Sharper diffraction patterns in FIG. 1(d) and FIG.1(e) are probably due to lower production of PEG-MOFs and a small amountof PEG-6000 penetrating the MOFs. In the case of PU-0.5, with a largerfraction of PEG-6000 penetrating the MOFs, the peaks in diffractionpattern are smaller, as depicted in FIG. 1(c).

Turning to FIG. 5A, FIG. 5B, FIG. 5C, and FIG. 5D, the morphology ofPU-1000-0.5, PU-4000-0.5, PU-6000-0.5 (PU-0.5), and PU-10000-0.5,respectively, is shown. Also, FIG. 6A, FIG. 6B, FIG. 6C, and FIG. 6Dshow the morphology of PU-1000-0.5, PU-4000-0.5, PU-6000-0.5 (PU-0.5),and PU-10000-0.5, respectively, at a higher magnification.

As seen in FIGS. 5A to 5D and FIGS. 6A to 6D, increasing molecularweight of PEG in PCM samples produces slightly larger particles. OnlyPU-0.5 produces very small PEG-MOFs, majorly of a uniform size andshape.

Referring to FIG. 7A and FIG. 7B, TEM image of UiO-66 alone and PU-0.5is shown, respectively. The MOF particles are connected through PEG-MOFnetworks as shown in FIG. 7B. The nanosized original PEG-MOF (smallercrystals) particles are clearly visible.

EXAMPLE 7

Thermal Storage Properties

The melting and freezing points and the latent heat of the composite PCMsamples were determined using a DSC-Q2000. DSC data were collected byheating 8.5 mg of each sample sealed in an aluminum pan. DSC data wereobtained under an argon gas flow rate of 20 mL/min at a heating rate of5° C./min. The maximal deviations for phase change temperature andlatent heat values determined based on the average of three measurementsare ±0.11° C. and ±0.43 J/g, respectively.

Referring to FIG. 8 , the melting-freezing DSC curves of (a) PU-0.5, (b)PU-0.7, (c) PU-1.0, (d) PU-1000-0.5, (e) PU- 4000-0.5, (f) PU-10000-0.5,(g) PU-0.5-CNT 5 wt. %, and (h) PU-NH₂-0.5) are shown. For allcomposites and pure PEG-6000, the melting temperature (T_(m)), freezingtemperature (T_(f)), latent melting heat, and latent freezing heat arelisted in Table 1.

TABLE 1 Phase-change characteristics of the PEG and PEG/UiO-66 ss-PEM[PEG-6000 = PEG] phase change temperature (° C.) phase change enthalpyΔH(J/g) Supercooling freezing melting freezing melting ΔT sample T_(f)(° C.) T_(m) (° C.) ΔH_(f) (J/g) ΔH_(m) (J/g) ΔT_(g) (° C.) R % E (%) γ(%) φ (%) PEG 39.5 63.84 201 221.3 24.34 100 PEG-0.5 36.6 58.03 135 14621.39 66 66.6 101 92.36 PU-0.7 34.2 56.67 130.2 134 22.42 63.5 64.1100.8 88.85 PU-1.0 37.4 58.12 113.1 122 20.76 55.7 55.7 101 77.18PU-0.5-CNT 42.2 61.69 119.1 120.9 19.53 52.1 54.2 104 76.48 PEG-100000.5/ 40.9 62.91 105 109.5 21.96 51.7 52 104 72.44 UiO-66 0.2 PEG-40000.5/ UiO-66 0.2 36.8 56.98 119.5 122.8 20.2 58.3 58.8 101 81.6 PEG-10000.5/ UiO-66 0.2 22.1 35.59 113.6 115.3 13.45 52.1 54.2 104 72.94PEG-6000.5/ 41.5 59.07 107.6 110 17.53 49.7 51.5 103.7 UiO-66 NH₂ 0.2T_(f) = freezing temperature, T_(m) = melting temperature, ΔH_(f) =latent heat in freezing process, ΔH_(m) = latent heat in meltingprocess, ΔT = supercooling, R % = impregnation ratio, E (%) =impregnation efficiency, γ = heat storage efficiency, and φ = energystorage ability (capability), and blank cells indicate that data are notavailable.

For the freezing and melting cycles, the enthalpies of PEG-6000 and thecomposites were determined using the region under the DSC curves. Asseen in Table 1, PU-0.5 performs better with an impregnation ratio of65.97% compared to 54.20% for PU-0.5-CNT 5 wt. %. The impregnationefficiency (E%) of PU-0.5 is also the highest among the tested samples.The thermal storage capacity of PU-0.5 composites indicates that almostall PEG-6000 molecules efficiently release/store energy through thetransition of phases. The supercooling ΔT values are based on thedifference between the freezing and melting temperatures, ΔTs of purePEG-6000, PU-0.5, PU-0.7, and PU-1.0 is 24.34° C., 21.39° C., 22.42° C.,and 20.76° C., respectively. The supercooling problem is overcome mostlikely due to the higher thermal conductivity.

A high latent heat value of 146 J/g, the highest among the testedsamples, was observed for the PU-0.5 sample. The high latent heat valueof PU-0.5 indicates that the ss-PCM is a potential candidate to producebuilding materials to provide comfort in buildings. Among the testedsamples, PU-0.5 also demonstrates the highest energy storage efficiencyof 96.48%. Further, the highest amount PolyMOF is formed in the case ofthe PU-0.5 sample which may be playing a vital role in enhancing thelatent heat storage capability.

EXAMPLE 8

Pore the Size and Pore Volume

The N₂ adsorption-desorption isotherms were obtained at −196° C. usingliquid N₂. The pore size distribution was calculated using theBarrett-Joyner-Halenda formula. The sample powders were evacuated for 3h at 200° C. Then the experiment was conducted at a heating rate of 5°C./min from room temperature to 600° C. under a dry nitrogen atmosphere.To assess the BET properties, i.e., pore size distribution, specificsurface area etc., a NOVA-1200 device (JEOL U.S.A.) was used. A TristarII 3020 system was employed to determine the BET surface area.

The N₂ adsorption-desorption isotherm of UiO-66, is shown in FIG. 9A.The isotherm of UiO-66 MOF shows Type I adsorption and desorptionfollowed by small loops for hysteresis of H3. UiO-66 has a calculatedBET surface area of 1067 m²/g which is indicative of the quite high porevolume. In FIG. 8B, a pore size distribution of UiO-66 is shown.

Typically, the performance of an ss-PCM depends on the porosity of thematrix which plays an important role in the polymer penetration process.The high pore volume of UiO-66 (micropore volume of 0.379 cm³g⁻¹ andpore diameter of 18.96 Å) and the structure of the polyMOF plays acrucial role in the high latent heat of the PCMs based on UiO-66. Theseproperties also increase the thermal stability of PU-0.5 PCM duringmelting and freezing cycles.

Referring to FIG. 9C, the N₂ adsorption-desorption isotherm of PU-0.5indicates that it is nonporous. The calculated BET surface area of thematerial is 2.80±0.18 m²g⁻¹ and the pore volume is 0.00137 cm³g⁻¹. Theseresults indicate that the pores in UiO-66 MOF are filled with PEG-6000,which is also confirmed by the PXRD data.

EXAMPLE 9

Thermal Stability

Thermogravimetric analysis (TGA) was performed using a Shimadzu thermalanalyzer (TA-50). About 10 mg each of the PCM samples was used toperform TGA. The heating rate was maintained at 5° C./min from 25 to600° C. under a dry nitrogen atmosphere.

Referring to FIG. 10 , TGA curve of PEG-6000, as-synthesized UiO-66, andPU-0.5 is shown. All the samples were dried at 100° C. for 24 h beforethe TGA test.

During the thermal decomposition, consecutive large weight losses occurfrom 220° C. to 460° C. The weight loss of porous UiO-66 is about 42.5%at 460° C. The successive weight losses can be due to dehydroxylation of—OH and the decomposition of the 1,4-benzene dicarboxylic acid and CO₂.At the end of the decomposition, the remaining residue is mainlyzirconium oxides derived from UiO-66. At about 78° C., pure PEG-6000starts to melt. Sudden decomposition of pure PEG-6000 sample begins atabout 400° C. and the total weight loss is 100% at about 440° C. Inaddition to the removal of absorbed water and hydroxyl groups from thematrix, the weight loss observed for PU-0.5 composite is due to thedecomposition of organic molecules. A noteworthy aspect is that thetotal weight of PU-0.5 is 17% higher than that of pure PEG-6000, clearlyindicating that PEG-6000 is thoroughly mixed and/or has penetrated theporous structure of UiO-66. Significant decomposition of PU-0.5 does nottake place below 80° C. The composite PU-0.5 PCM demonstrates thethermal stability of the combination of UiO-66 and PEG-6000. By creatinga self-protective barrier, the porous UiO-66 support can enhance thethermal stability of organic PEG-6000. The overall weight loss of thePU-0.5 composite is 78.2% at about 560° C. In the lower temperaturerange of 25° C. to 200° C., the weight loss of PU-0.5 is negligible.

EXAMPLE 10

Thermal Cycling Properties

The thermal cycling performance of the composite PU-0.5 PCM, which isimportant for determining its feasibility of commercial use, was alsoevaluated. PU-0.5 PCM is thermally stable even after 200 cycles and aDSC curve was recorded every 50 cycles. The DSC curves recorded duringcycling of PU-0.5 PCM are depicted in FIG. 11 . Both exothermal andendothermal curves do not significantly change with cycling, indicatingthat the composite has a stable life cycle with good thermalreliability. Multi-cycle DSC curves recorded during cycling of PU-0.5PCM indicate that PEG-6000 can sustain phase changes in terms oftemperature and enthalpy, consistently. PU-0.5 can be used to store andrelease latent heat at a constant temperature over multiple cycles. Thehigher melting and solidification latent heats of PU-0.5 is due to theincreased absorption of PEG-6000 promoted by capillary forces.Furthermore, it is noteworthy that during the melting cycle, formationof any vapor/gas was not observed, and no voids were not created duringthe freezing process. The thermal cycling performance of a composite PCMis critical for its commercial viability. Also, SEM images and FTIRspectra obtained after 200 cycles were similar to those of the startingmaterials.

EXAMPLE 11

Seepage Test

The seepage test was performed using a sample disk and powders ofUiO-66, PEG-6000, PU-0.5, and PU-0.5-CNT-5 wt. %. The UiO-66 powder didnot melt after 10 min and even after 20 min. The same phenomenon wasobserved for the PU-0.5 sample. The microstructure of the PU-0.5composite remained unchanged indicating the absence of leakage and evenat 80° C. for 20 min, PU-0.5 powder did not melt. The weight of thePU-0.5 sample before and after heat treatment remained unchanged,indicating the absence of any weight loss. No leaks were observed in theleakage test performed for the PU-0.5-CNT 5 wt. % sample either.

EXAMPLE 12

Thermal Conductivity

Thermal conductivity of the sample powders was determined using circulardisk samples with a TCi Conductivity Analyzer, Canada. The equipmentuses modified transient plane source (MTPS) and the measurement methodis based on C-Therm Technologies.

The thermal conductivity of UiO-66 alone is 0.84 W/mK, while the thermalconductivity of PEG-6000 alone is 0.22 W/mK. A relatively high thermalconductivity of 0.52 W/mK was observed for the PU-0.5 PCM. This resultis very promising considering that material with a higher conductivityhas a high demand in the field of solar energy storage and itsapplication can enhance the charging and discharging rates.

The invention claimed is:
 1. A solution method for making a phase changematerial, comprising: ultrasonically agitating a mixture comprisingpolyethylene glycol (PEG), an alcoholic solvent and an UiO-66metal-organic framework (MOF) to form a composite; evaporating thealcoholic solvent from the composite to form the phase change material(PCM), wherein a weight ratio of PEG:UiO-66 MOF in the PCM is 0.5:0.2 to1.0:0.2, wherein the PCM has first particles and second particles,wherein the first particles comprise PEG and the UiO-66 MOF and aresmaller than the second particles, wherein the second particles compriseonly the UiO-66 MOF and have a longest dimension of 200 nm to 500 nm,wherein the first particles and second particles have a roundedoctahedral shape, and wherein the second particles are connected througha network of the first particles.
 2. The method of claim 1, wherein thealcoholic solvent is ethanol; and the PEG has an average molecularweight of from 4000 to
 10000. 3. The method of claim 2, wherein the PEGhas an average molecular weight of
 6000. 4. The method of claim 1,wherein the PCM has a thermal conductivity of from 0.8 to 0.9 W/mK; andthe PCM has an energy storage efficiency of from 92% to 97%.
 5. Themethod of claim 1, wherein the PCM has a latent heat value of from 125J/g to 175 J/g at a PEG:UiO-66 weight ratio of 0.5, and the PCM has athermal conductivity of from 0.4 W/mK to 0.6 W/mK at a PEG:UiO-66 weightratio of 0.5.
 6. The method of claim 1, wherein the mixture furthercomprises carbon nanotubes (CNTs).
 7. The method of claim 6, wherein thePCM comprises the CNTs in an amount of from 1 wt % to 4 wt % of the PCM.8. The method of claim 1, wherein the PCM has an impregnation ratio ofat least 60%.
 9. The method of claim 1, wherein the PCM has: a freezingtemperature of at least 35° C.; and a melting temperature of at least55° C.
 10. The method of claim 1, wherein the PCM has an impregnationefficiency of at least 55%.
 11. The method of claim 1, wherein the PCMhas a heat storage efficiency of at least 99%.
 12. The method of claim1, wherein the PCM has an energy storage ability of at least 70%. 13.The method of claim 1, wherein the PCM has a latent heat in a freezingprocess of at least 100 J/g.
 14. The method of claim 1, wherein the PCMhas a latent heat in a melting process of at least 110 J/g.
 15. Themethod of claim 1, wherein the mixture further comprises an aminegroup-containing compound.